"I would love to have her endorsement," Rubio said in a previous meeting with The Post and Courier's editorial board. "I'm not sure she's going to endorse anyone."

Fellow candidate and former Florida Gov. Jeb Bush (R) told NBC News in an interview before Haley's decision that her endorsement "would be the most powerful, meaningful one in the state."

He added that, were he not to get Haley's endorsement, it would send "a signal that I got to work harder."

Haley, a rising star in the party who delivered the GOP's response to US President Barack Obama's State of the Union speech last month, previously said that she was unsure if she'd endorse any of the candidates, although she had been tough on GOP front-runner Donald Trump.

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On Tuesday, she said that Trump represented "everything a governor doesn't want in a president." And, in her State of the Union response, she alluded to Trump when she said, "During anxious times, it can be tempting to follow the siren call of the angriest voices. We must resist that temptation."

Rubio is currently polling third in South Carolina at 16%, according to the RealClearPolitics average of several polls. He trails Sen. Ted Cruz (R-Texas), who is at 18%, and Trump, who is at 35%.